Broadband optical beam steering system and device

ABSTRACT

A small, compact optical scanning system with small aperture size requirements, wide field-of-regard and minimal color dispersion characteristics. The inventive scanning system and method provides for optical beam steering over a broad spectral band and over a wide field-of-regard. The inventive system includes a novel device for receiving an input wavefront of electromagnetic energy along a first axis and refracting the wavefront as an output wavefront along a second axis. The device is a unique form of a multi-channel liquid crystal array in which each channel has a length parallel to the first axis. By applying signals to change the refractive index of each channel, the incident wavefront can be steered at an angle with respect to the first axis and otherwise manipulated according to the index variant pattern induced in the array. Accordingly, the output beam is steered in response to the applied signals.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to infrared and optical systems.More specifically, the present invention relates to systems and methodsfor effecting steering of infrared and optical beams.

[0003] 2. Description of the Related Art

[0004] For many applications, there is a need to generate imagesoptically. Military and commercial aircraft applications, for example,utilize systems which transmit and/or receive infrared, near-infrared,optical or other electromagnetic energy. The input and/or output beamsmust be steered in a desired pointing direction.

[0005] For beam steering, the prior art includes steering mirrors,pointing gimbals and monochromatic electro-optical, beam steeringmechanisms. Steering mirrors require output windows many times the sizeof the system optical entrance pupil to scan over a large field ofregard. Unfortunately, the mirror form factor requirements greatlyincrease the size of the sensor package.

[0006] The gimbaled approach involves use of an imaging system mountedin a dome that is gimbaled to provide a desired pointing angle. Thegimbals must point the entire sensor to scan the field-of-regard.Unfortunately, for aircraft applications, this requires a mirror belowthe platform line, which necessitates a hole in the platform. Inaddition, the dome and optical assembly is bulky, typically requiresconsiderable volume, and has a radar cross-section which tends toincreases the observe-ability of the vehicle.

[0007] The monochromatic electro-optical, beam steering approachinvolves the use of a liquid crystal as the manipulated medium. Thisapproach relies on a diffractive grating pattern in a liquid crystalarray. Displacing the grating causes a phase delay that steers the beam.Unfortunately, this approach only operates effectively for monochromaticor near-monochromatic light sources. For non-monochromatic lightsources, this approach causes undesirable color dispersion.

[0008] Accordingly, a need exists in the art for small, compact opticalscanning system with small aperture size requirements, widefield-of-regard and minimal color dispersion characteristics.

SUMMARY OF THE INVENTION

[0009] The need in the art is addressed by the beam steering system andmethod of the present invention. The invention provides a means foroptical beam steering over a broad spectral band and over a widefield-of-regard in a small, compact optical scanning system with smallaperture size requirements, wide field-of-regard and minimal colordispersion characteristics.

[0010] The inventive system includes a novel device for receiving aninput wavefront of electromagnetic energy along a first axis and forrefracting the input wavefront as an output wavefront along a secondaxis. The device is a unique form of a liquid crystal array which can beelectrically manipulated to change the effective refractive index ofeach pixel. The index of refraction of the device varies in response toan applied voltage. The voltage is supplied by a microprocessor and/or aservo-control system. By changing the index, the incident phase frontcan be steered at an angle with respect to the first axis and otherwisemanipulated according to the index variant pattern induced in the array.Accordingly, the output beam is steered in response to the appliedvoltage.

[0011] In the illustrative implementation, the device is an array ofliquid crystal devices. Counter-rotating optical wedges are provided forrestoring color balance to the output wavefront. In the illustrativeembodiment, a mirror is included for compensating the wavefront outputby said first and second counter-rotating optical wedges. The wavefrontreflected by the mirror may be output by an imaging lens or othersuitable device.

[0012] In accordance with the present teachings, beam steering isaccomplished through a refractive variation not a diffractive one. Thisallows the spectral bandwidth to be much broader than for adiffractively manipulated phase wavefront.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a diagram which illustrates an illustrative applicationa typical conventional gimbaled optical scanning system.

[0014]FIG. 2 is a diagram which illustrates the field-of-regard of thetypical conventional gimbaled optical scanning system of FIG. 1.

[0015]FIG. 3 is a diagram showing an illustrative application of theoptical beam steering system of the present invention.

[0016]FIG. 4 is an isolated view of the optical beam steering system ofthe present invention with associated control circuitry.

[0017]FIG. 5 is a diagram of the liquid crystal array utilized in theoptical beam steering system of the present invention.

[0018]FIG. 6 is a sectional side view of a single liquid crystal channelof the array of FIG. 5.

[0019]FIG. 7 is a sectional side view of multiple liquid crystalchannels of the array of FIG. 5.

[0020]FIG. 8a is an end view of multiple liquid crystal channels of thearray of FIG. 5.

[0021]FIG. 8b is a sectional side view of multiple liquid crystalchannels of the array of FIG. 5 showing an arrangement by which leadsare brought out of the cells.

[0022]FIG. 9 is a diagram which illustrates that the beam steeringdevice of the present invention may be used for multiple laser frequencysteering.

DESCRIPTION OF THE INVENTION

[0023] Illustrative embodiments and exemplary applications will now bedescribed with reference to the accompanying drawings to disclose theadvantageous teachings of the present invention.

[0024] While the present invention is described herein with reference toillustrative embodiments for particular applications, it should beunderstood that the invention is not limited thereto. Those havingordinary skill in the art and access to the teachings provided hereinwill recognize additional modifications, applications, and embodimentswithin the scope thereof and additional fields in which the presentinvention would be of significant utility.

[0025]FIG. 1 is a diagram which illustrates an illustrative applicationof a typical conventional gimbaled optical scanning system. Theconventional scanning system 10′ includes a gimbaled mirror 12′ (notshown in FIG. 1) mounted within a dome 13′ gimbaled in an aircraftplatform 14′. The mirror 12′ scans a beam 16′ through a window 18′.

[0026]FIG. 2 is a diagram which illustrates the field-of-regard of thetypical conventional gimbaled optical scanning system of FIG. 1. Asillustrated in FIG. 2, the gimbaled mirror 12′ receives the beam 16′through an entrance pupil 19′ over a field of regard θ. Clearly, asmentioned above, this use of steering mirrors requires an output window18′ many times the size of the system optical entrance pupil 16′ to scanover a large field of regard θ. Unfortunately, for aircraftapplications, this requires necessitates a large hole in the platform14′. In addition, the dome 13′ and associated optical assembly is bulky,typically requires considerable volume, and has a radar cross-sectionwhich tends to increases the observe-ability of the vehicle.

[0027] Hence, a need exists in the art for small, compact opticalscanning system with small aperture size requirements, widefield-of-regard and minimal color dispersion characteristics.

[0028]FIG. 3 is a diagram showing an illustrative application of theoptical beam steering system of the present invention. The beam steeringsystem of the present invention 10 includes a device 20 which allows forelectronic or electro-optical refractive steering of an input or outputbeam 16. As discussed more fully below, in the illustrativeimplementation, the device is a broadband liquid crystal array. The beam16 is refracted through an window 18 which is approximately equal, indimension, to the entrance pupil 19 of the device 20.

[0029]FIG. 4 is an isolated view of the optical beam steering system 10of the present invention with associated control circuitry. Asillustrated in FIG. 4, the refracted beam 16 is corrected for dispersionby a dispersion arrangement 22 consisting of first and secondcounter-rotating wedges 24 and 26. The diameter of the wedges 24 and 26is determined by the diameter of the beam 16. The wedge angles aredetermined by the color dispersion inherent in the material in thedevice 20 through which the beam propagates for the spectral componentsof the beam.

[0030] The orientation of the wedges 24 and 26 is controlled by aservo-control system 28, which operates under command of a controller30. The controller 30 may be implemented with a microprocessor,application specific integrated circuit, programmable digital logiccircuit or other suitable circuit as will be appreciated by those ofordinary skill in the art. The controller may operate under command of asystem controller 32, which would receive input from and provide outputto a user via an interface 34.

[0031] As discussed more fully below, the controller 30 applies avoltage to the beam steering device 20, which determines the angle atwhich the beam is refractively steered thereby. Simultaneously, thecontroller supplies signals to the servo-controller 28 to makeorientational adjustments to the wedges 24 and 26 required to correctfor any color dispersion caused by the device 20.

[0032] The dispersion corrected beam is directed to a mirror 36. Themirror 36 is a component of a compensating group 37 which includes alens 38. The mirror 36 compensates for any small angular change in thebeam 16 caused by the dispersion group 22. As will be appreciated bythose skilled in the art, the flatness and spectral reflectance of themirror are primary design considerations. The mirror should be as flatas practical for the application. The spectral reflectance will be setby the reflective coatings selected for the application.

[0033] The lens serves to focus the beam 16 to a detector, camera oreyepiece 40.

[0034]FIG. 5 is a diagram of the liquid crystal array 20 utilized in theoptical beam steering system 10 of the present invention. As shown inFIG. 5, the broadband beaming steering device 20 consists of amulti-channel array of channels 21.

[0035]FIG. 6 is a sectional side view of a single liquid crystal channelof the array 20 of FIG. 5. As depicted in FIG. 6, each channel 21 of thearray 20 is filled with a liquid crystal formulation 25 and acts as awaveguide. Application of a voltage across each channel 21 induces achange in refractive index of the liquid crystal material. Thisintroduces a phase delay in the beam 16 as it propagates through eachchannel.

[0036]FIG. 7 is a sectional side view of multiple liquid crystalchannels of the array 20 of FIG. 5. By varying the relative voltagechannel-to-channel across the array, a variation between the channelindices is created. As depicted in FIG. 7, the variation between channelindices applies a phase delay across an incident optical wavefront 16 tocreate an electro-optical wedge. The optical wedge can be manipulatedelectro-optically to scan the field-of-regard. The optical wedge of theliquid crystal component is manipulated by varying the applied voltage.The rotating wedges correct the residual dispersive color that isinherent in any optical medium. This dispersive color correction is asmall angular component compared to the large angular scan generated bythe liquid crystal component.

[0037]FIG. 8a is a cross-sectional end view of multiple liquid crystalchannels 21 of the array 20 of FIG. 5. As shown in FIGS. 6 and 8a, eachchannel 21 is created by etching a cavity in a substrate 33 to create acell 31. The cell size should be kept small to ensure single modeoperation. In the illustrative embodiment, the cell thickness (channellength) is several centimeters and the operating temperature T=23° C.The substrate 33 may be any suitable etchable material. In theillustrative implementation, the cells are 3×3 microns and are separatedwith 1 micron of substrate.

[0038] A conductive material is flashed into the floor of the cell 31 toprovide a ground 27. Next, in the illustrative implementation, the cell31 is filled with a liquid crystal material to provide a medium 25.Those skilled in the art will appreciate that any material can be usedas a medium so long as it refracts a beam of electromagnetic energy inresponse to an applied voltage. In the illustrative embodiment, asolution of 1% PTTP-15 dissolved in methylene chloride was chosen as theliquid crystal material for the medium 25. A voltage strip 29 ofconductive material is applied as a cover for each cell. The voltagestrip may be common in some embodiments. However, in most conservativeembodiment, each cell has its own unique circuit and therefore, does notact as a common ground.

[0039] When a voltage is applied between the voltage strip 29 and theground 27, a capacitive field is created therebetween. The field linesup the molecules of the medium and induces a rotation in thepolarization of electromagnetic energy (e.g., light or infrared energy)propagating therethrough. Transmittance is maximized in the individualchannel by creating either 1) total internal reflection by index cavityindex selection or 2) by coating the interior with a reflective film.

[0040] A filter 23 is provided at the input aperture of each channel. Anarray (not shown) of conductors would be provided to allow for selectiveapplication of an applied voltage to an individual cell.

[0041]FIG. 8b is a sectional side view of multiple liquid crystalchannels of the array of FIG. 5 showing an arrangement by which leadsare brought out of the cells. Each row has a layered insulated wiringrouting as shown. The dimensions are exaggerated to illustrate the path.Each of the row outputs are staggered in space to avoid physicalinterference when organized along the column.

[0042]FIG. 9 is a diagram which illustrates that the beam steeringdevice of the present invention may be used for multiple laser frequencysteering.

[0043] Thus, the present invention has been described herein withreference to a particular embodiment for a particular application. Thosehaving ordinary skill in the art and access to the present teachingswill recognize additional modifications, applications and embodimentswithin the scope thereof.

[0044] It is therefore intended by the appended claims to cover any andall such applications, modifications and embodiments within the scope ofthe present invention.

[0045] Accordingly,

What is claimed is:
 1. A system for steering a beam of electromagneticenergy comprising: a beam steering device for receiving an inputwavefront of electromagnetic energy along a first axis and refractingsaid input wavefront as an output wavefront along a second axis at anangle with respect to said first axis in response to control signals,said beam steering device comprising: a two-dimensional array of liquidcrystal channels, wherein the channels have a length which is parallelto the first axis, and contacts for a control signal for each of saidchannels; and a controller for providing said control signals.
 2. Thesystem of claim 1, wherein each of said channels is a single modeoptical waveguide.
 3. The system of claim 1, wherein the beam steeringdevice comprises a substrate and a channel is created by etching acavity in said substrate and filling said cavity with liquid crystalmaterial.
 4. The system of claim 3, wherein said the index of refractionof said liquid crystal material within a channel varies in response tothe control signal applied to said channel.
 5. The system of claim 4,wherein each channel comprises a conductive material flashed on one wallof the channel to provide a ground and a second strip of conductivematerial applied to an opposite wall of the channel to provide a controlsignal contact.
 6. The system of claim 1, further comprising anarrangement for restoring color balance to said output wavefront, saidarrangement including first and second counter-rotating optical wedges.7. The system of claim 6, further comprising a mirror for compensating awavefront output by said first and second counter rotating opticalwedges.
 8. The system of claim 7 further comprising an imaging lens. 9.A beam steering device for receiving an input wavefront ofelectromagnetic energy along a first axis and refracting said inputwavefront as an output wavefront along a second axis at an angle withrespect to said first axis in response to control signals, said beamsteering device comprising: a two-dimensional array of liquid crystalchannels, wherein the channels have a length which is parallel to thefirst axis, and contacts for a control signal for each of said channels.10. The system of claim 9, wherein each of said channels is a singlemode optical waveguide.
 11. The system of claim 9, wherein the beamsteering device comprises a substrate and a channel is created byetching a cavity in said substrate and filling said cavity with liquidcrystal material.
 12. The system of claim 11, wherein said the index ofrefraction of said liquid crystal material within a channel varies inresponse to the control signal applied to said channel.
 13. The systemof claim 12, wherein each channel comprises a conductive materialflashed on one wall of the channel to provide a ground and a secondstrip of conductive material applied to an opposite wall of the channelto provide a control signal contact.